1. Field of Invention
The present invention relates to a semiconductor and a method for manufacturing a semiconductor. More particularly, the present invention relates to a high-voltage device and a method for manufacturing a high-voltage device.
2. Description of Related Art
A high voltage device is one of the most important devices utilized in a highly integrated circuit. Erasable programmable read only memory (EPROM) and flash memory are two of the high-voltage devices most often used in computers and electronic products.
Due to the increasing number of semiconductor devices incorporated in integrated circuits, the size of transistors needs to be decreased. Accordingly, as the channel length of the transistors is decreased, the operating speed is increased. However, the short channel effect caused by the reduced channel length is becoming serious. If the voltage level is fixed as the channel length is shortened, the strength of the electrical field is increased according to the equation, electrical field=electrical voltage/channel length. Thus, as the strength of the electrical field increases, the energy of electrons increases and electrical breakdown is likely to occur.
In the conventional high-voltage device, the formation of an isolation layer is used for the purpose of increasing the channel length. Hence, the high-voltage device is able to work normally at a high electrical voltage.
FIG. 1 is a schematic, cross-sectional view of a conventional high-voltage device. As shown in FIG. 1, a field oxide layer 102 is located on a P-type substrate 100. A gate oxide layer 103 is located on the P-type silicon substrate 100. A gate electrode 104 is located on the field oxide layer 102 and the gate oxide layer 103. A source region 106 and a drain region 108 are located in the P-type substrate 100. An N.sup.- -type doped region 112 is located in the substrate beneath the drain region 108, the field oxide layer 102 and a portion of the gate electrode 104. A P-type doped region 114 is located under the source region 106 and a portion of the gate electrode 104.
In order to increase the breakdown voltage of the high-voltage device, it is necessary to decrease the dopant concentration of the drift region, which is the dopant concentration of the N.sup.- -type doped region 112. However, the current-driving performance and the channel conductivity between the source region 106 and the drain region 108 under the gate electrode 104 in the substrate 100 are decreased.
Additionally, when the manufacturing technique is promoted to a sub-quarter micron level, for example, a line width of 0.18 microns or less, it is difficult to decrease the typical design rule of the high-voltage device.